The United States openly accused Russia on Thursday of sending combat forces into Ukraine and threatened to tighten economic sanctions, but Washington stopped short of calling Moscow’s intensified support for separatist forces an invasion.

President Barack Obama again ruled out US military action, saying there must be a diplomatic solution. But he said he would reaffirm US commitment to Nato allies in the region at a summit next week.

“We are not taking military action to solve the Ukrainian problem. What we’re doing is to mobilize the international community to apply pressure on Russia,” he told reporters at the White House.